Mathematical instrument.



R. O. GOXHEAD.

MATHEMATICAL INSTRUMENT.

APPLICATION FILED OGT.25,1909.

Patented Mar. 7, 1911.

RALPH C. COXHEAD, OF OAKLAND, CALIFORNIA.

MATHEMATICAL INSTRUMENT.

Specification of Letters Patent.

Patented Mar. '7, 1911.

Application flied October 25, 1908. Serial No. 524,372.

To all whom it may concern:

Be it known that I, RALPH C. CoxnEAn, a citizen of the United'States,residing at Oakland, in the county of Alameda and State of California,have invented certain new and useful Improvements in MathematicalInstruments, of which the following is a specification.

The hereinafter described invention is designed for use in the drawingof circles and circular lines generally, and more particularly for usein connection with class rooms for describing circular lines on theblack board in the working out of geometrical problems; the object beingto provide a simple and effective device for accurately describingcircular lines on the black board without injury to the surface thereof,the instrument embracing adjustable means for positioning the same forthe drawing of circles of varying diameter, the said adjustable meansbeing of such construction as to hold onto the surface of the blackboard to prevent slippage of the instrument while drawing circles andcircular or curved lines.

To comprehend the invention reference should be had to the accompanyingsheet of drawings, wherein for the post 6.

Figure 1 is a perspective view of the instrument positioned for thedrawing of circles or circular lines. Fig. 2 is a detail sectional viewof the adjusting and holding means. Fig. 3 is a similar view of thechalk holding thimble, illustrating the spring clamp therein forengaging the chalk and the adjusting screw and follower for moving thestick of chalk outwardly.

In the drawings, the numeral 1 is used to designate a base plate,constructed of any suitable material, which, in the present case, isformed with a longitudinally disposed central slot 2. \Vithin thisslotted portion of the base plate is slidably held the screwthreadedstud 3, the same carrying an enlarged head 4,-which bears against theunder face of the base plate 1, and, in conjunction with the circularplate 5, acts as a clamp The plate 5 fits over the screw threaded stud 3and bears onto the upper face of the base plate, being held clampedthereto by means of the wing nut 7, which works on the screw-threadedstud 3. Through the said. stud 3 loosely extends the reduced end 8 ofthe post 6, the same screwing into the stem 9, carrying a suction disk10. To the upper end of the post 61 is ator may readily hold the same.

On the upper face of the base plate 1 and to one side of thelongitudinally .disposed slot 2, is formed a graduated scale 12,preferably in inches, ranging from one to twelve inches, so that thepost 6 may be set at any desired point and there clamped' to the baseplate by screwing tight the wing nut 7, which causes the plates 4 and 5to firmly clamp onto the base plate. For instance, if a circle twelveinches in diameter is to be drawn, the post 6 will be set to the pointdesignating six inches, the position of the post being varied inaccordance with the distance of the line, from the center from whichitis to be struck. The

secured a timer knob 11, so that the ope screw stud 3 is held againstrotation within the slotted portion of the base plate, by reasonof theflattened faces 13 thereof bearing against the walls of the slottedportion 2.

At the outer end 'of the base plate is held a chalk container, thimbleor shell 14, the lower end of which is fitted. within an enlarged openportion of the base plate 1 until its shoulderedportion 15 bears ontothe upper face of the base plate, there being screwed onto theprojecting end thereof a securing washer 16. Within the container,thimble or shell 14 is located a spring clamp holder 17, which receivesand clamps a stick 18 of chalk, The inner end of the stick of chalkbears against a follower 19, movable within the spring clamp holder, theextension 20 of which projects through a vertical slot formed in thewall of the container, thimble or shell 14, and the said extension 20 isengaged by an adjusting screw 21, and by means of which the follower 19is lowered to force outwardly the stick 18 of chalk to compensate forthe wear thereof. To the head of the container, thimble or shell 14 issecured a finger knob 22, which, preferably, is free to turn as theinstrument is swung around. The chalk is thus held out of contact withthe fingers of the operator, thereby maintaining the same clean whileusing the instrument.

In operation, the post 6 is first adjusted on the base plate to thediameter of the circle to be described, and the sucker disk 10 thenpressed onto a point on the black board previously determined as thecenter of the circle to be described, the suction of said disk beingsubstantially sufficient to support the weight of the instrument. The

post 6, the said clamp plates servir operator with one hand grasps theknob 11 and with the other the knob 22, exerting slight pressure ontothe post 6. and by the hold on knob 22 swinging the base plate 1 5 todescribe the desired circle, or are of a circle, the chalk pencil 18,which is in contact with the surface of the board producing the circleline. During this movement, the clamp plates 4 and 5 turn freely on,the

as a pivot joint for the base plate to swing on. The describedinstrument is designed to take the place of the string and chalk commonly employed in class-rooms for describing circles and circular linesin the working out of geometrical problems. Inasmuch as the disk 10holds to the surface of the blackboard by suction, there is nodisfigurement to the surface of the board by the use thereof, whichfollows from the use of such devices as employ centering studs which cutinto the surface of the board.

lVhile preference is given to the use of a suction disk as a bearinghead for holding the instrmnent centered, still, any form of a base diskwhich will prevent slippage of the base plate may be employed as acentering head.

The described instrument will readily appeal to instructors inmathematics, inasmuch as it insures the pupils producing on the surfaceof the board perfectly drawn circular lines.

Having thus described the invention what is claimed as new and desiredto be protected by Letters Patent is 1. An instrument for the describedpurpose, the same comprising a slotted base plate having its uppersurface provided with a scale of measure, a suction disk slidably heldto said base plate, adjustable means for clamping the said disk andholding the same in locked position relative to -the base plate, aholder for a chalk stick secured to the outer end of the base plate,

and means within the holder for feeding outwardly the stick of chalkheld therein.

2. In an instrument for the described purpose, the combination with aslotted base plate provided on one face thereof with an indicatingscale, a stud slidably mounted within said base plate, a post extendedthrough and rotatably held within the slidable stud, a suction disksecured to the lower end of the said post, and means for clamping theslide stud in adjusted posit1on relative to the base plate.

3. In an instrument for the described purpose, the same comprising aswinging base plate, a suction disk for holding the same to a board,-slidable clamping means for adjusting the suction disk and holding thesame in locked position, a chalk container secured to the outer end ofthe slotted swinging base plate, a spring holder within the container,and means for feeding downwardly the chalk.

4. In an instrument for the described purpose, the combination with aslotted swinging base plate, of a chalk container secured to the outerend thereof, a spring clamp holder within the container, a followertherein, and means outside of the container for forcing downwardly thefollower to regulate the position of the chalk stick, and adjustablesuction means for holding the base plate to the surface of a board.

of the base plate, a chalk holder carried at the outer end of theswinging base plate, and means therein for feeding downwardly a stick ofchalk held within the said holder.

6. In an instrument for the described purpose, the combination with aswinging base plate, a suction disk carried thereby for frictionallyholding the same to the surface of a board, adjustable clamping meansfor regulating the position of the suction disk relative to the baseplate, a loose connection between the suction disk and the clampingmeans for permitting free swinging movement of the base plate, a chalkholder carried at the outer end of the base plate, and means within thechalk holder for regulating the outward feed of the chalk containedtherein.

In testimony whereof I have signed my name to this specification in thepresence of two subscribing witnesses.

, RALPH G. COXHEAD. Witnesses:

EZRA W. DECOTO, N. A. AOKER.

